feat: Allow for dev packages to be dynamically loaded

[schloerke]

Summary

Add support for local package development and function-based app initialization in
AppDriver$new(app_dir=). This allows testing Shiny apps without requiring package
installation and provides more flexible ways to initialize test apps.

Key Features

1. Local Package Development Support: When testing apps within a package,
   library(<pkg>) or require(<pkg>) calls automatically execute pkgload::load_all()
   to load the local package's source code instead of requiring the package to be
   installed first.

Package installation is no longer required

2. Function-Based App Initialization: app_dir= now accepts a function that
   returns a Shiny app object or runs an app directly, providing more flexibility in test
   setup.

Examples

Testing a Shiny app in a package without installing

Using a function to initialize your app

Example 1: Function returning a Shiny app object

test_that("App works with custom initialization", {
   app <- AppDriver$new(
     app_dir = function() {
       library(mypackage)  # Automatically loads source code if in package
       create_my_app(config = "test")
     }
   )
   # ...
 })

Example 2: Function for golem/rhino apps

 test_that("Golem app works", {
   app <- AppDriver$new(
     app_dir = function() {
       library(mygolemapp)
       run_app()  # Returns shiny.appobj
     }
   )
   # ...
 })

 Example 3: Package function directly provided
```r
 test_that("Custom app runner", {
   app <- AppDriver$new(run_app)
   # ...
 })

Example 4: Running app directly in function

  test_that("Custom app runner", {
    app <- AppDriver$new(
      function() {
        library(shiny)
        runApp(
          test.mode = TRUE, 
          shinyApp(
            ui = fluidPage("Hello"),
            server = function(input, output) {}
          )
        )
      }
    )
    # ...
  })

Technical Details:

• Disabled on CRAN checks (testthat::is_checking()) to ensure only installed packages
  are used during R CMD check
• Automatically detects when a function's environment belongs to the local package and
  upgrades it to load the package first
• Shims library() and require() in the background R process to intercept package
  loading
• When testing from within the package root, automatically disables Shiny's R/ folder
  autoloading to prevent conflicts

Benefits

• Faster development cycle: Test changes without reinstalling the package
• CI/CD friendly: Works seamlessly with GitHub Actions workflow updates
• Framework agnostic: Supports vanilla Shiny, golem, rhino, and other frameworks
• Backward compatible: Existing tests continue to work without modification

---

The magic bits of this PR can be found at https://github.com/rstudio/shinytest2/pull/402/files#diff-1e9839b0ee9b578e1094ebabe21a38467f417789f5b3e17fec51556b6706a851R90 where we use {pkgload} to load a local R package before calling library() or require(). This is done to mimic Shiny app behavior, not testthat testing behavior. I want the author to be required to call library() or require() for the app code to be the same both within shinytest2 testing and when run normally.

TODO:

• Test local R package
• Test golem app
• Test rhino app

[schloerke]

Merging to move forward with a release soon